This invention relates to a shroud segment for a turbomachine, in particular an aircraft engine, with a flow duct and a bladed rotor wheel arranged in the flow duct which perform a relative rotary movement and are flown by a hot working gas, with the shroud segment comprising a metallic substrate provided by the flow duct and a ceramic rubbing coating which is bonded to the inner surface of the substrate and is abradable by the blade tips for the formation of a narrow sealing gap.
On turbomachinery, for example aircraft engines or stationary gas turbines, the sealing gap between the tips of the rotor blades fitted to a rotor and the inner surface of a casing surrounding these rotor blades must be minimized to achieve high efficiency. As is known, the required minimum sealing gap width is achieved in that the ceramic coating with low thermal conductivity applied to the inner surface of the metallic casing for the purpose of thermal insulation of the latter equally acts as wearaway or attrition coating, hereinafter referred to as rubbing coating. In order to achieve the required wearaway or rub, a burning-out material, for example a polymer powder, is, in a known version, admixed to the ceramic material, this powder providing for the porosity of the rubbing coating and, thus, for the release of free particles from its surface as the blade tips move relatively to the casing. In the case of stationary blades, the ceramic rubbing coating can also be applied to a rotary engine component. Other than the above mentioned shroudless rotary blade tips and their mating stationary ceramic-coated shroud segments, stationary blades can, at their tips, be equipped with shrouds with—abradable—ceramic rubbing coating.
Such a rubbing coating, i.e. a segmented, abradable ceramic coating applied to the metallic substrate of a flow duct segment, is described in detail in Specifications U.S. Pat. Nos. 6,102,656 and 5,705,231. It comprises an underlayer of zirconium oxide stabilized with ceroxide, magnesium oxide, yttrium oxide or calcium oxide to which, in this sequence, an intermediate layer and an abradable top layer for the formation of the sealing gap is applied, these layers again including a heat-insulating ceramic zirconium oxide as a base material.
For automatic formation of the gaps, reduction of the surface temperature of the components facing the blade tips and the protection of these components against overheating, highly heat-insulating, but abradable coatings on the basis of zirconium oxide are equally used in various other publications, for example U.S. Pat. Nos. 5,530,050, 5,866,271 or 5,997,248, with this material having a very low coefficient of thermal conductivity, forming a thick-walled layer and being composed or structured in at least the area facing the blade tips such that, together with the blade tips, a rubbing surface for the formation of a minimum sealing gap between the stationary and the rotary components is created.
Zirconium-based rubbing coatings are, however, disadvantageous because of their tendency towards sintering and phase transformation in a temperature range above 1200° C., as a result of which the ceramic material embrittles and its adhesion on the metallic substrate is impaired. Even if the gas turbine with zirconium-based rubbing coating operated in this elevated temperature range is cooled by means of a cooling medium passed along the rear side of the metallic substrate, embrittlement and separation of the heat-insulating abradable ceramic material applied with a specified, sufficiently large coating thickness cannot be mitigated even with high cooling flow rates—as demonstrated by tests.